Increase scope of pause api

[lbonn]

Now succeeds at all time: pauses current download as well as the api queue.

I still need to write some tests

[OYTIS]

Looks great!

I feel like it might be useful if Aktualizr::Pause() didn't return until the download (if there is one in progress) is "really" paused, i.e. the image or the metadata about the image is committed to the storage. But that might be another user story.

[eu-siemann]

At first glance looks great to me as well!
As we just have discussed with @OYTIS it should be fine for Pause to return immediately. In another PR we should remove the Shutdown call and introduce Abort instead. So the user who wants to terminate the application while being sure that all data is safe must call Abort. Abort will block until the Download operation saves the state and exits and all other commands are removed from the API queue.

[codecov-io]

Codecov Report

Merging #1075 into master will increase coverage by 0.09%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master    #1075      +/-   ##
==========================================
+ Coverage   75.37%   75.47%   +0.09%     
==========================================
  Files         158      158              
  Lines        9262     9263       +1     
==========================================
+ Hits         6981     6991      +10     
+ Misses       2281     2272       -9

Impacted Files	Coverage Δ
src/libaktualizr/primary/events.h	100% <ø> (ø)	⬆️
src/libaktualizr/primary/sotauptaneclient.h	100% <ø> (ø)	⬆️
src/libaktualizr/primary/results.h	100% <ø> (ø)	⬆️
src/hmi_stub/main.cc	0% <0%> (ø)	⬆️
src/libaktualizr/primary/reportqueue.h	27.27% <0%> (-6.07%)	⬇️
src/libaktualizr/primary/sotauptaneclient.cc	86.43% <100%> (+0.03%)	⬆️
src/libaktualizr/primary/aktualizr.h	100% <100%> (ø)	⬆️
src/libaktualizr/primary/reportqueue.cc	100% <100%> (ø)	⬆️
src/libaktualizr/primary/aktualizr.cc	84.34% <100%> (+1.17%)	⬆️
src/libaktualizr/uptane/fetcher.cc	96.87% <100%> (ø)	⬆️
... and 1 more

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[lbonn]

Significant changes since last review.

I also removed DownloadPaused and DownloadResumed events.